1. Field of the Invention
The present invention relates to apparatuses and methods for monitoring, measuring, or analyzing the sag of a weight material in a drilling fluid.
2. Description of Relevant Art
A drilling fluid, or xe2x80x9cmudxe2x80x9d which a drilling fluid is also often called, is a specially designed fluid that is circulated in a wellbore or borehole as the wellbore is being drilled in a subterranean formation to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation.
A drilling fluid typically comprises water and/or oil or synthetic oil or other synthetic material or synthetic fluid as a base fluid, with solids in suspension. A non-aqueous based drilling fluid typically contains oil or synthetic fluid as a continuous phase and may also contain water dispersed in the continuous phase by emulsification so that there is no distinct layer of water in the fluid. Such dispersed water in oil is generally referred to as an invert emulsion or water-in-oil emulsion.
A number of additives may be included in such drilling fluids and invert emulsions to enhance certain properties of the fluid. Such additives may include, for example, emulsifiers, weighting agents, fluid-loss additives or fluid-loss control agents, viscosifiers or viscosity control agents, and alkali. Weighting agents are commonly added to increase the density of the fluid. Barite is a typical weighting agent, although other minerals are also common.
Suspensions of solids in non-vertical columns are known to settle faster than suspensions in vertical ones, due to the xe2x80x9cBoycott effect.xe2x80x9d This effect is driven by gravity and impeded by fluid rheology, particularly non-Newtonian and time dependent rheology. Manifestation of the Boycott effect in a drilling fluid is known as xe2x80x9csag.xe2x80x9d Sag may also be described as a xe2x80x9csignificantxe2x80x9d variation in mud density ( greater than 0.5 lbm/gal) along the mud column, which is the result of settling of the weighting agent or weight material and other solids in the drilling fluid.
Drilling fluid in deviated wellbores can exhibit the Boycott effect, and sag, in both static and dynamic situations. In this context, static is a totally quiescent fluid state, such as when drilling has ceased; dynamic is any situation where the fluid is exposed to a shear stress, such as for example during drilling. Sag can result in formation of a bed of the weighting agent on the low side of the wellbore, and stuck pipe, among other things. In some cases, sag can be very problematic to the drilling operation and in extreme cases may cause hole abandonment.
U.S. Pat. No. 5,086,646, issued Feb. 11, 1992 to Jamison et al., teaches an apparatus and method for analyzing well fluid sag, particularly static sag. Dynamic sag, however, can be more than an order of magnitude greater than static sag. As directional drilling and deviated wellbores become more common if not the norm in the oil and gas industry, more and improved apparatuses and methods are needed to measure or analyze dynamic sag.
An apparatus and method are provided for measuring or analyzing dynamic sag as well as static sag in a drilling fluid or other solids bearing fluid. The apparatus of the invention comprises a tube or other elongated container and a rotatable, concentric inner cylinder or shear shaft. The tube and shear shaft are assembled together such that the annulus region between them is capable of holding fluid, especially fluid to be tested. Said shear shaft is preferably free to rotate, and its rate of rotation is preferably controllable. A rotator or other means is provided for rotating the shaft. For example, rotation of the shaft might be provided by coupling a motor or motor drive mechanism to an end of the tube and shear shaft assembly. Preferably, the motor should be engageable and disengageable from the shaft so that the motor drive mechanism can be disengaged from the assembly during the measurement process to preferably avoid any interference with the measurement.
The apparatus of the invention further comprises a holder or housing, such as a pressure vessel, for holding the assembly comprising the tube and the shear shaft such that the axis of the tube and shear shaft is positioned at an angle to vertical, such as about 45 degrees, for example. Preferably, said holder may also be filled with fluid, such as a pressurization fluid, in a manner that immerses or covers the tube and shear shaft assembly. Preferably, such holder fluid can be heated and pressurized to simulate conditions in a wellbore penetrating a subterranean formation. At least one seal or other isolator isolates or otherwise keeps separate or apart the test fluid and the pressurization fluid. Such seal is preferably positioned on at least one end of the tube, and in one embodiment may preferably be positioned on the same end of the tube (or tube and shear shaft assembly) as a coupler for attaching a motor drive mechanism to the shear shaft (or to the tube and shear shaft assembly). The top or higher end of the tube and shear shaft assembly, which is preferably the opposite end (rather than the same end) from any motor drive mechanism, contains one or more magnets, preferably rare-earth permanent magnets.
Further, the apparatus of the invention comprises a support for the tube and shear shaft assembly preferably positioned at or near the center of the assembly. The support should allow rotation, preferably frictionless rotation, about the horizontal axis normal to the tube and shear shaft assembly axis. The support should also be comprised of (or be associated with) an electrically conductive medium. A preferred example of a particularly suitable support comprises two cross-spring pivots.
At least one and preferably two pistons may be included in the apparatus on the same axis as the pivots or other support. The pistons accommodate expansion and contraction of the test fluid due to changes in temperature and pressure, thereby allowing the test fluid to be heated, cooled and pressurized without changing the center of mass of the test fluid relative to the pivot axis due to thermal expansion or fluid compressibility.
Energizing external coils, preferably in a Helmholtz coil configuration, are arranged external to the holder or pressure vessel, at the magnet end of the tube and shear shaft assembly to provide a magnetic field, preferably a uniform magnetic field. Coil control circuitry is associated with the energizing coils to enable detection and measurement of change in torque or moment about the pivot axis of the assembly. Any detector capable of detecting and measuring change in torque or moment about a central axis or pivot point could be substituted for the coils and coil circuitry, provided the detector is sufficiently sensitive to detect sag.
In the method of the invention, the apparatus of the invention or another apparatus capable of conducting the steps of the method is used for measuring or analyzing the dynamic and/or static sag of a fluid such as a drilling fluid or other fluid used in drilling, cementing, casing, or workover operations in a subterranean wellbore. As used herein, the term xe2x80x9cdrilling fluidxe2x80x9d shall be understood to include fluids used in any of these downhole operations. In the method, when measuring or analyzing dynamic sag, shear is applied to a test sample of the fluid at a controlled rate and the rate of sag or settling by the weighting agent (or other material) in the fluid is measured. When measuring or analyzing static sag, shear is not applied to the test sample of the fluidxe2x80x94the shear shaft is not rotatedxe2x80x94and the rate of sag or settling by the weighting agent (or other material) in the fluid is measured.
During either a dynamic or static test, the fluid is in an inclined tube assembly. Settling of the weighting agent or material causes the center of mass to change, which in turn causes a change in the torque or moment about the pivots or other holders for the tube and shear shaft assembly. The resultant moment is preferably measured by energizing external coils such that the upper end of the tube (which comprises a permanent magnet) and shear shaft assembly is driven back to its initial zero position. This zero position is initially detected by closing a circuit attached to a stationary contact and to the tube and shear shaft assembly (preferably at the upper end). The coil control circuitry is a self-integrating error accumulator that automatically drives the coil current. As the coil is precisely energized, it provides a force on the permanent magnet, which repositions the tube and shear shaft assembly at zero position. When compensated for the minor effects of temperature and other effects, the coil current is proportional to the imbalance of the tube and shear shaft assembly. The greater the imbalance, the greater the coil current, indicating the greater the amount of sag in the fluid. Measurement of the sag may then be calculated as a function of the current over time.